The trend toward a finer line width has become more pronounced in recent years in the fields related to the design and manufacture of VLSI circuits. It is therefore becoming more difficult to form fine lines of required width by methods of reduction projection and exposure based on the use of conventional ultraviolet light.
Electron-beam direct exposure is a method that meets requirements for such finer exposure patterns. The electron-beam direct exposure method features electron-beam spots finely focused by reduction lenses. Pattern exposure is accomplished by a so-called tracing method. A resulting shortcoming is that the throughput and the manufacturing efficiency are lower than those achieved with methods involving reduction projection and exposure.
Electron-beam direct exposure equipment operating on a partial one-shot exposure principle has recently been developed in order to overcome this shortcoming.
FIG. 8 depicts the structure of an electron-beam direct exposure apparatus operating on the partial one-shot exposure principle. As shown in FIG. 8, the electron-beam direct exposure apparatus comprises an electron gun 11 for emitting an electron beam EB, and a first aperture 12 provided with a quadrilateral opening. The electron-beam direct exposure apparatus also comprises a second aperture 15 that allows the electron beam passing through the first aperture 12 to be transformed to a prescribed shape and size. The second aperture 15 is therefore provided with a quadrilateral variable-shape opening 16 as a conventional means of electron-beam direct exposure. At the same time, the second aperture 15 is provided with a plurality of partial one-shot exposure openings 17 whose shape corresponds to a portion of the pattern to be exposed.
When exposure is actually performed using a conventional electron-beam direct exposure method, the cross-section of the electron beam EB emitted by the electron gun 11 is initially changed to a quadrilateral shape by the first aperture 12. The electron beam EB that has acquired the quadrilateral shape is then directed by a variable-shape deflector 13 toward the area of the variable-shape opening 16 in the second aperture 15. At this time, the electron beam strikes the second aperture 15, and the portion of the electron beam that has passed through the variable-shape opening 16 is directed toward the wafer 20.
Consequently, adequately correcting the position in which the electron beam EB strikes the second aperture 15 allows the shape of the electron beam EB passing through the second aperture 15 to be changed to any beam spot size. The electron beam EB shaped by the variable-shape opening 16 is reduced by a reduction lens 18 and is ultimately directed by a deflector 19 toward any point on the wafer 20.
The partial one-shot exposure openings 17 formed in the second aperture 15 will now be described. During partial one-shot exposure, the electron beam EB formed by the first aperture 12 is directed by a selecting deflector 14 toward any of the partial one-shot exposure openings 17 in the second aperture 15. The electron beam EB that has been formed in accordance with the shape of the partial one-shot exposure openings 17 in the second aperture 15 is reduced by a reduction lens 18 and is directed by the deflector 19 toward any point on the wafer 20.
Exposure performed using such partial one-shot exposure openings 17 allows target patterns to be exposed by being irradiated with a single electron beam EB. Consequently, providing the second aperture 15 in advance with openings that correspond to various patterns makes it possible to reduce the number of exposure shots in comparison with cases in which patterns with complex shapes are exposed through a variable-shape opening 16. As a result, the throughput is markedly increased.
During actual partial one-shot exposure, the surface area of the second aperture 15 is limited. Therefore, it is difficult in practical terms to form partial one-shot exposure openings that would fit all types of patterns to be exposed. Consequently, partial one-shot exposure openings are formed in the second aperture only for the patterns that are repeated to a certain extent, with parts of the patterns removed in advance. Such repeating patterns are exposed by partial one-shot exposure, and any nonrepeating patterns in contact with these patterns are exposed using the variable-shape opening 16.
However, the following problems are encountered when the wafer 20 is exposed by electron-beam direct exposure in such a manner. Specifically, partial one-shot exposure is used on the repeating portions of various patterns, and the variable-shape opening 16 is used to perform exposure on nonrepeating portions, so patterns obtained by different exposure methods come into contact with each other. As a result, dimensional differences (line width or line length) exist between the patterns in the areas where patterns obtained by exposure through the variable-shape opening 16 are connected with patterns obtained by partial one-shot exposure. The reason is that dimensional differences exist between the beam spot formed on the wafer 20 by the variable-shape opening 16 and the beam spot produced by partial one-shot exposure. A resulting drawback is a reduction in the reliability of an LSI device manufactured by this method.
The reasons causing the aforementioned dimensional differences will now be described in detail. When performing exposure through the variable-shape opening 16, it is possible to carry out a prescribed correction prior to the actual exposure step. To accomplish this, a reference mark or the like used for correction purposes is first placed on the stage that carries the wafer 20. This reference mark is scanned with a beam spot of prescribed size. The dimensions of the actual beam spot can be determined based on the reflected electron signal generated. It is thus possible to constantly maintain accurate beam spot dimensions by correcting these dimensions when they vary due to fluctuations or the like in the ratio to which the electron beam has been reduced by a reduction lens.
By contrast, partial one-shot exposure involves preforming patterns as partial one-shot exposure openings 17 in the second aperture 15. Therefore, it is sometimes impossible to obtain accurate beam spot dimensions that accord with design values on the wafer 20 due to fluctuations or the like in the manufacturing precision attained during the formation of the openings 17 for partial one-shot exposure or in the ratio to which the electron beam EB is reduced by the reduction lens 18. In such cases it is difficult to accurately measure the beam spot dimensions of a plurality of patterns projected onto the wafer 20. As a result, it is impossible to correct the dimensions of a beam spot produced by a partial one-shot exposure opening, and line width errors or the like occur in connected portions in the manner described above.
When, for example, a pattern whose line width design dimension is 0.20 micrometer is exposed using a variable-shape opening 16 and a partial one-shot exposure opening 17 (as shown in FIG. 9), the size of the beam spot produced by the variable-shape opening 16 can be accurately obtained at a level of 0.20 micrometer by the above-described correction procedure. By contrast, the beam spot dimensions of the electron beam EB obtained using the partial one-shot exposure opening 17 are governed by the opening dimensions of the second aperture 15, the reduction ratio of the reduction lens, and the like, making it impossible to correct the beam spot dimensions.
FIG. 9 depicts a case in which the pattern dimensions provided by the partial one-shot exposure opening exceed the design values by 10%.